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Full text of "The Cambridge Natural History"

vrr                              GEOGRAPHICAL RANGE                             I2/
the Marsupials as primitive creatures. It implies on the whole
that the Marsupials have sprung from a stock with an allantoic
placenta. The alternative is to assume the independent develop-
ment of an allantoic placenta in both groups of the Mammalia;
unless indeed the genus Penrameles is to be held to be the most
primitive race of Marsupials living, a hypothesis which does not
appear on the face of it likely. So long as it was believed that
the mammary pouch of the Monotremes was the equivalent of
the marsupium of the Marsupials, the persistence of this structure
seemed to be a bond of union between the groups. But it is now
known that the marsupiurn is a special organ confined to the
Marsupials, an argument which is rather in favour of their being
a lateral development of the mammalian stem. It is to be re-
marked also that the marsupium is feeblest in the Polyproto-
doiits, which may perhaps be looked upon as the most primitive
of the Marsupials, owing to their more numerous teeth and other
points to be referred to immediately.
Not only are the Marsupials interesting from the point of
view of their structure ; their present and past distribution is of
equal interest. During the Mesozoic epoch they occurred in
Europe and North America; but not, so far as negative evidence
means anything, in Australia, which is now their headquarters.
In Europe Marsupials lingered on into the Tertiary period, when
they finally became extinct. In America, of course, the group
has persisted to the present day. Now it is important to notice
that the two main subdivisions of the Marsupials, the Poly-
protodontia and the Diprotodontia, exist to-day in both Australia
and South America. These two divisions, it should be explained,
differ principally in that one has numerous, the other rarely
more than two,1 incisors in the lower jaw. It is perhaps the
more widely distributed opinion that the Polyprotodontia are the
more archaic group ; this opinion rests upon one or two facts in
addition to the absence of specialisation in the incisor teeth.
Among the Polyprotodontia the total number of teeth is greater—
a clearly primitive character; secondly, the general form of the
body of these animals, with four subequal limbs and carnivorous
or omnivorous diet, contrasts with the purely vegetarian and much
specialised Kangaroos at any. rate. Finally—and sufficient stress
1 When there are more than two, two are especially developed.    See Figs. 76, 77
(pp. 149, 150).